The present invention relates to a cathodic protection method and apparatus for protecting a metal structure exposed in the atmosphere such as exterior wall of a building, roof, and bridge from being corroded by oxidation, and to as an application thereof, a cathodic protection method and apparatus of a steel member contacted with water in water piping or tanks.
A metal structure exposed in the atmosphere, even when its metal surface is painted, with the passage of time, is oxidation corroded by electrolytic water and dissolved oxygen in that water formed on the surface by rain water or moisture in the air or pollution substances in the atmosphere or the like.
As protective anode and cathodic protection system for performing corrosion protection of the coated surface of such a metal structure, there is an apparatus developed and commercialized by C. L. I. Systems Inc. (U.S.A.) (U.S. Pat. No. 2,579,259 (Japanese Patent Laid-open Publication No. 4-318183).
This electrical corrosion protection apparatus has an effect for corrosion protection of a metal structure having a surface protected by surface coating which is exposed in the atmosphere, however, has the following two unavoidable problems. These problems are
(1) Current control means as a subject matter has a function to apply a current proportional to the environmental humidity detected by a humidity sensitive device. However, in practice, depending on the amount of sea salt particles or atmospheric pollution substances dissolved in the water film, even with the same humidity and the same thickness of the water film, it is evident that electrical resistance per unit length of the water film itself is greatly varied. Therefore, since the current necessary for corrosion protection is changed with the quality of the water film, even when the same magnitude of current is applied, reaching range of corrosion protection current is changed according to the environmental water quality at that time.
Further, the surface coating film of the metal structure is partly degraded by sand dust or other environmental affecting factors, generating a defect in part thereof. If this defective part expands, even with the same environmental humidity, the current amount necessary for extensive surface corrosion protection of the metal structure containing the defective part is greatly increased. For example, assuming a case where a large amount of sea salt particle is dissolved in the water film and the defect of the coating film is generated around the protective anode of the apparatus, at a humidity of 60 to 70% and a low temperature, and if only a small amount of current is supplied, since the supplied current flows into the above-described defective part, the current is consumed in a small area in the vicinity of the protective anode, thus reducing the reaching range of the corrosion protection current.
(2) To prevent over-corrosion protection of the metal structure in the vicinity of the protective anode, the anode output voltage is set to a maximum of 12V. However, as described above, when the area of the defective part of coating film is increased, to achieve the desired corrosion protection range, it is necessary to further increase the voltage to increase the current. For this requirement, current application by a maximum output voltage of 12V cannot meet.
With the above prior art circumstances, it is therefore an object of the present invention to provide a cathodic protection method and apparatus for a metal structure in which the area of corrosion protection obtainable by a single anode can be increased to a maximum without causing over-corrosion protection.
With the aim of solving the above problems, the inventors have conducted the following experiments and investigations and accomplished the present invention.
In the experiments, various systems have been constructed, trial and error has been repeated to confirm whether or not the desired function can be achieved, and initially the system as shown in FIG. 1 has been completed.
In the figure, numeral 1 indicates a terminal of a 100 to 200V power supply, 2 is a fuse, 3 is a surge current absorption varistor. 4 is a transformer which steps the output down to 18 to 20V. Further, 5 is a rectifier circuit which converts AC to DC. 6 and 7 are capacitors. 8 is a regulator which controls the voltage applied to a main anode 15. 9 and 10 are capacitors, and 21 is also a capacitor. 22 is a regulator which controls the voltage applied to a pilot anode 20 to a constant value. 23 and 24 are capacitors.
The above-described power supply terminal 1 to capacitor 10, capacitor 21, regulator 22, and capacitors 23 and 24 are incorporated in a controller 30.
A transistor 11, resistors 12 and 14, and a Zener diode 13 constitute a first current limitation means 100 for limiting the current supplied to the main anode 15 to a predetermined value. Further, a capacitor 25, an operational amplifier 26, resistors 27, 28 and 29 constitute a second current limitation means 101 which detects the current from the pilot anode 20 to a coated steel plate (metal structure) 16 to be corrosion protected, and according to the detected current value, controls the output voltage of the regulator 8 through the ground of the regulator 8, whereby flowing a predetermined optimum current from the main anode 15 to the coated steel plate 16. Between the main anode 15 and the coated steel plate 16, a special medium 70 having an electrical resistance is disposed.
The first current limitation means 100 is integrated with the main anode 15, and the second current limitation means 101 is integrated with the pilot anode 20. In the figure, numeral 17 indicates a coating film provided on the coated steel plate 16 to be corrosion protected. 18 is a defective part of the coating film 17, and 19 denotes a water film formed on the surface of the coated steel plate 16.
In the apparatus of the above construction, when the output voltage of the regulator 22 is a constant value of 8V to 12V, and the current is increased by an increase in electrical conductivity of the water film 19 and expansion of the defective part 18, accordingly the output voltage of the regulator 8 is increased from 10V to, for example, 15V to supply the desired optimum corrosion protection current to the coated steel plate 16.
FIG. 2 shows an outline of a test apparatus for confirming the corrosion protection function of the protection apparatus of the construction shown in FIG. 1. The main anode 15 and the pilot anode 20 are stuck to the coated steel plate 16 through an insulating two-sided bonding material. Anode wires 30a from the controller 30 are connected to the corresponding main anode 15 and pilot anode 20, and further, a cathode wire 30b connected to a base material 32 of the coated steel plate 17.
Part of the coating film at a position away from the anode connection position of the coated steel plate 16 is peeled out to form an artificial coating film defective part 18 of 10 mm in diameter. The coating film defective part 18 is provided with an Ag/AgCl microelectrode (xcfx86=0.1 mm) 31 coated with agar agar containing saturated KCl. Potential of the steel plate base material 17 to the electrode 31 is outputted to a computer through a buffer to collect data.
The controller 30 is connected with AC 100V power supply, after confirming that the voltage of the pilot anode 20 is constant at the setting value 10V, the coated steel plate 16 is subjected to an exposure test for about 30 days. As a result, in the state of a humidity of less than 60%, the voltage applied to the main anode 15 exhibits the minimum value 10V, the potential increases with increasing humidity, reaching about 13V in rainfall.
Further, in addition to the above-described position, a coating film defective parts are also formed between the coating film defective part of the above position and the respective anodes of the main anode 15 and the pilot anode 20, so that the voltage applied to the main anode 15 in rainfall is the maximum of 15V.
In this state, the potential of the coating film defective part 18 shows a value of about xe2x88x92850 mV, showing that a sufficient corrosion protection is achieved.
Further, no sign of over-corrosion protection is noted in the coating film 17 in the vicinity of the main anode 15, and since the current flowing from the main anode 15 to the defective part 18 of the coating film 17 is sufficiently high, it can be conjectured that the voltage in the coating film in the vicinity of the main anode 15 is lower than the voltage of generating over-corrosion protection by a voltage drop in the water film on the surface of the insulating bonding material (special medium) 70 in the lower part of the main anode 15.
That is, when the required corrosion protection current is increased by increasing other coating film defective parts between the coating film defective part 18 at a position away by 2.5 to 3.0 m from the main anode 15 and the main anode 15, in the prior art, the main anode voltage 12V becomes a limit, and the potential of the coating film defective part 18 is not decreased to the corrosion protection potential, on the other hand, in the method and apparatus according to the present invention, the voltage is increased with increasing required current value, and therefore, it is confirmed that even if coating film defective parts are substantially increased, the potential of the farthest defective part 18 is sufficiently decreased to attain the corrosion protection potential. From this fact, a new technology superior in corrosion protection capability to the prior art has been established by the present invention.
As a most economical method for performing corrosion protection of a metal structure by a combination of a coating film formation paint and a cathodic protection method, a test as shown in FIG. 2 has been carried out using an apparatus of the cathodic protection method by the present invention, in which a metal structure (coated steel plate) is coated with a general-purpose insulating paint as an underlayer, and a paint of low electrical resistance is provided as a top layer.
Using a paint having a resistance of 0.2 xcexa9cm as a top layer paint, an artificial defective part of coating film is provided at a position 5 m away from the main anode 15, the controller 30 is energized, water is sprayed on the surface of the coating steel plate 16, and the potential of the coating film defective part 18 is measured.
The potential shows a value of xe2x88x92850 to xe2x88x92950 mV, which is within the passivation area, and shows that corrosion protection is sufficiently achieved.
Next, as a most economical method for performing corrosion protection of a metal structure by a combination of the coating film formation paint and a cathodic protection method, to further enhance the function of the above-described method, the following method has been tested. As the coating of the metal structure, the underlayer uses a general-purpose insulating paint, the intermediate layer uses a conductive paint, and the top layer uses a paint having a good weather resistance. First, on the underlayer coating film coated on the surface of the metal structure, a thin-plate anode is stuck with an insulating tape, on top thereof, the above intermediate layer conductive paint and the top layer paint of superior weather resistance are coated, and the test similar to the above is carried out.
As the conductive paints, using a carbon-based type material having a volume resistivity of 0.9 xcexa9xc2x7cm and a nickel-based type material having a volume resistivity of 0.0025 xcexa9xc2x7cm, an artificial defective part of coating film is provided at a position 10 m away from the main anode 15, a voltage is applied, water is sprayed on the artificial defective part and the potential of the defective part is measured.
The potential shows about xe2x88x92850 mV for the carbon-based type material, and in the case of the nickel-based type material, even when the anode voltage is further decreased to 1 to 2V, a value of xe2x88x92900 to xe2x88x921100 mV is shown, which is within the passivation area, obviously showing that sufficient corrosion protection is achieved.
In a color steel plate using an ordinary insulating paint, corrosion protection potential of the artificial defective part is reached only to about 2.5 to 3.5 m away from the main anode, whereas by the optimum combination of the paint and cathodic protection, a technology capable of remarkably increasing the corrosion protection range can be established.
When considering a vehicle such as an automobile as the metal structure, for cathodic protection for a vehicle, there is a product developed and commercialized also in Japan (U.S. Pat. No. 2,579,259) by C. L. I. Systems Inc. (U.S.A.).
This electrical corrosion protection apparatus provides an effect of corrosion protection of metal parts of a vehicle, in which the anode voltage is 12V by the limitation of the battery voltage. Therefore, to further enhance the corrosion protection ability, it is necessary to increase the voltage, for example, in the results of comparative test of the potential of coating film defective part at a position 1.5 m away from the anode shown in FIG. 3, for the case of the anode voltage of 15V, the potential falls within the corrosion protection area at a relative humidity of higher than 61%, whereas for the case of 12V, corrosion protection area is not achieved unless the relative humidity is more than 66%.
On the other hand, the inventors, as a system for performing electrical corrosion protection of a vehicle body, by an electronic circuit shown in FIG. 4, in which voltage from a 12V battery power supply is increased to 15V by a DC/DC converter and applied to the anode through a low current mechanism. In FIG. 4, a battery indicated by numeral 201 is used as a power supply, 12V voltage is applied to the system. 202 includes a battery protection circuit and an IC protection circuit. 204 is a DC/DC converter, which steps up and controls the output voltage, for example, to 20V. 205 is an overvoltage prevention system, which serves to decrease the voltage when the voltage is unusually increased over the above controlled value. Further, by a system comprising a transistor 207, a resistor 208 and a Zener diode, as an example, the voltage is controlled to a constant voltage of 15V and supplied from constant current systems 213A to 213D. 15V voltage is applied from the constant current systems 213A to 213D to anodes 214A to 214D through wiring. Up to the circuit elements 202 to 213 are incorporated in a single case as one unit as a whole. The anodes 214A to 214D are adhered onto the coated surface under the vehicle, thereby protecting the iron structure inside the coating film by the cathodic protection method.
As an example of this system, an improved system for suppressing current consumption, accomplished through trial and error, is shown in FIG. 5 to FIG. 7.
In FIGS. 5 to 7, using the battery 201 as the power supply, 12V voltage is applied to the system. Numeral 202 includes a battery protection circuit and an IC protection circuit. An 8-pin IC 217 has an oscillation function, converts DC into pulses, the voltage is increased by a voltage converter 229, and controlled at 20V by a FET 228. The system comprising the resistor 215 and the Zener diode 216 supplies power for operating the 8-pin IC 217. The voltage is regulated by a system comprising the capacitors 232, 234, 235 and the diode 233 and supplied to the transistor 207. Resistor containing transistors 223 and 225 serves an overvoltage prevention function. By the system comprising the transistor 207, the resistor 208 and the Zener diode 209, the voltage is regulated, for example, to a constant voltage of 15V, which is supplied to the constant current systems 213A to 213D. From the constant current systems 213A to 213D to the anodes 214A to 214D through wiring, a voltage of, for example, 15V is applied. Elements from the battery and the IC protection circuit 202 to the constant current systems 213A to 213D are incorporated in a single case as a control unit. The anodes 214A to 214D are adhered to the coated surface under the vehicle, thereby protecting the iron structure inside the coating film by the cathodic protection method.
Using the system of the above construction, a corrosion protection system for vehicle with minimized current loss is obtained in which the battery voltage is stepped up by an IC system without using a transformer.
In FIGS. 4 and 5, symbols 203, 206, 210, 218, 219, 220, 232, 235, and 238 are capacitors. Further, symbols, 208, 212, 215, 221, 222, 224, 226, 227, 230, 231, 236, 237, and 240 are resistors. Still further, 209, 216, and 239 are Zener diodes. Yet further, 236 is a FET, 223 and 235 are resistor containing transistors, and 229 is a voltage converter.
Further, details of a circuit 500 in FIG. 5 are shown in FIG. 6. In FIG. 6, symbols 241, 242, 246, 247, 248, 249, and 250 are resistors, 243 is a capacitor, 244 is a transistor, and 245 is an operational amplifier.
Still further, details of the constant voltage systems 213A to 213D are shown in FIG. 7. In FIG. 7, 252 and 254 are resistors, 251 is a transistor, and 253 denotes a Zener diode.
As the metal structure subjected to the method of the present invention, metal structures such as buildings and bridges, vehicles such as automobiles, metal pipings which will be described later, metal washing tanks for washing foods such as vegetables are included, the method of the present invention can be applied to all types of structures if those are required to be corrosion protected.
As can be seen from the above description, the cathodic protection method for a metal structure according to one embodiment of the present invention is a cathodic protection method of flowing a current from an external power supply to the metal structure to protect the metal structure from corrosion characterized in that a main anode and a pilot anode are mounted on a coating film of the metal structure, a cathode is mounted to a metal base material of the metal structure, a predetermined voltage is applied from the pilot anode to the metal structure, a magnitude of corrosion protection current of the metal structure to be corrosion protected is read from a current value of the pilot anode varying with variation of corrosion environment of the metal structure, the application voltage of the main anode is increased or decreased in accordance with the current value, whereby supplying an optimum corrosion protection current according to corrosion environment of the metal structure.
The cathodic protection method for a metal structure according to another embodiment of the present invention is wherein the pilot anode and the main anode are not only one to one correspondence, but also one pilot anode corresponds to a plurality of main anodes.
The cathodic protection method for a metal structure according to another embodiment of the present invention is wherein the pilot anode and the main anode are mounted to the metal structure to be corrosion protected through an insulating two-sided bonding material, whereby a water film is formed continuously from the surface of the metal structure to the respective anode, so that when a corrosion condition of the metal structure is met, a current optimum for corrosion protection flows between the anode and the cathode connected to the metal base material of the metal structure.
Further, the cathodic protection apparatus for a metal structure according to another embodiment of the present invention is a cathodic protection apparatus of flowing a current from an external power supply to the metal structure to protect the metal structure from corrosion characterized by comprising a main anode and a pilot anode mounted on a coating film of the metal structure, a cathode connected to a metal base material of the metal structure, a predetermined voltage is applied from the pilot anode to the metal structure, current control means for applying a predetermined voltage from the pilot anode to the metal structure, reading a magnitude of corrosion protection current of the metal structure to be corrosion protected from a current value of the pilot anode varying with variation of corrosion environment of the metal structure, increasing or decreasing the application voltage of the main anode in accordance with the current value, whereby supplying an optimum corrosion protection current according to corrosion environment of the metal structure.
The cathodic protection apparatus according to claim 5 of the present invention is wherein the main anode and the pilot anode are not only one to one correspondence, but one pilot anode corresponds to a plurality of main anodes.
The cathodic protection apparatus according to another embodiment of the present invention is further comprising an insulating two-sided bonding material for mounting the pilot anode and the main anode on the surface of the metal structure, by mounting the respective anodes to the metal structure to be corrosion protected through the insulating two-sided bonding material, whereby a water film is formed continuously from the surface of the metal structure to the respective anode, so that when a corrosion condition of the metal structure is met, a current optimum for corrosion protection flows between the anode and the cathode connected to the metal base material of the metal structure.
The cathodic protection apparatus according to another embodiment of the present invention is wherein functions of the pilot anode and the main anode are integrally provided.
The cathodic protection apparatus according to another embodiment of the present invention is wherein the current control means comprises transistors, diodes, operational amplifiers, resistors, and other electronic components.
Further, the cathodic protection method for a metal structure according to another embodiment of the present invention is wherein the metal structure is a vehicle, and as a voltage application source, a combination of a battery incorporated in the vehicle with electronic components for increasing the voltage of the battery is used.
Further, the cathodic protection apparatus according to another embodiment of the present invention is wherein the metal structure is a vehicle, and as a voltage application source, a combination of a battery incorporated in the vehicle with electronic components for increasing the voltage of the battery is used.
Yet further, the cathodic protection method for a metal structure according to another embodiment is characterized in that a coating of a metal structure to be corrosion protected is performed by a combination of an underlayer of an ordinary insulating paint and a top layer of a conductive paint, and to the coated metal structure.
Yet further, the cathodic protection method for a metal structure according to another embodiment is characterized in that a coating of a metal structure to be corrosion protected is performed by a combination of an ordinary insulating paint as an underlayer, a weather resistant paint as a top layer, and a conductive paint as an intermediate layer, and to the coated metal structure.
Yet further, the cathodic protection method for a metal structure according to another embodiment of the present invention is a cathodic protection method of flowing a current from an external power supply to the metal structure to protect the metal structure from corrosion characterized in that the metal structure is a water pipe, an anode is mounted to inside of the water pipe, a cathode is mounted to a metal base material of the water pipe, and a voltage is applied between the anode and the cathode, thereby flowing a predetermined current in the water pipe.
The cathodic protection method for a metal structure according to another embodiment of the present invention is wherein polarity of the anode and the cathode is reversed with the passage of time, thereby dissolving a film formed on the surface of the electrode with the passage of time.
Further, the cathodic protection apparatus for a metal structure according to another embodiment of the present invention is a cathodic protection apparatus of flowing a current from an external power supply to the metal structure to protect the metal structure from corrosion characterized by comprising an anode mounted to the inside of a water pipe as the metal structure, a cathode mounted to a metal base material of the water pipe, and current control means for applying a voltage between the anode and the cathode, thereby flowing a predetermined current in the water pipe.
The cathodic protection apparatus for a metal structure according to another embodiment of the present invention is wherein the current control means has a function to reverse polarity of the anode and the cathode with the passage of time to dissolve a film formed on the surface of the electrode with the passage of time.
Further, the cathodic protection method for a metal structure according to another embodiment of the present invention is a cathodic protection method of flowing a current from an external power supply to the metal structure to protect the metal structure from corrosion characterized in that the metal structure is a metal tank, an anode is mounted to the inside of the metal tank and a cathode is mounted to a metal base material of the metal tank, a voltage is applied between the anode and the cathode, thereby flowing a predetermined current in the metal tank.
The cathodic protection method for a metal structure according to another embodiment of the present invention is wherein polarity of the anode and the cathode is reversed with the passage of time, thereby dissolving a film formed on the surface of the electrode with the passage of time.
Further, the cathodic protection apparatus for a metal structure according to another embodiment of the present invention is a cathodic protection apparatus of flowing a current from an external power supply to the metal structure to protect the metal structure from corrosion characterized by comprising an anode mounted to the inside of a metal tank as the metal structure, a cathode mounted to a metal base material of the metal tank, and current control means for applying a voltage between the anode and the cathode, thereby flowing a predetermined current in the metal tank.
The cathodic protection apparatus for a metal structure according to another embodiment of the present invention is wherein the current control means has a function to reverse polarity of the anode and the cathode with the passage of time to dissolve a film formed on the surface of the electrode with the passage of time.